Method of making an article of a titanium alloy by plastically deforming at room temperature and/or polishing

ABSTRACT

A method of making an article of a titanium alloy is disclosed, which includes i) quenching a work piece, which is made of a titanium alloy composition containing a) about 0.01-5 wt % Bi based on the weight of the composition; b) at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having a temperature higher than a beta transition temperature of the titanium alloy composition to a temperature lower than 500° C. at a cooling rate greater than 10° C./second between the beta transition temperature and 500° C., so that the quenched work piece contains beta phase as a major phase; and ii) plastically deforming the quenched work piece or polishing the quenched work piece so that a surface of the polished work piece has an average surface roughness less than about 0.1 μm.

FIELD OF THE INVENTION

The present invention relates to a method of making a titanium alloyarticle, and more particularly to a method of making a titanium alloyarticle by plastically deforming at room temperature and/or polishingthe surface thereof to has an average surface roughness less than about0.1 μm. The titanium alloy article can be a dental casting or a medicalimplant.

BACKGROUND OF THE INVENTION

Due to its lightweight, high strength-to-weight ratio, low elasticmodulus, superior chemical corrosion resistance, and excellentmechanical properties at high temperature up to 550° C., titanium andits alloys have been widely used on aerospace, chemical, sports, andmarine industries. Their superior biocompatibility also makes them idealas the primary materials used in dental and osteological restorations orimplants, such as artificial bone pins, bone plates, shoulders, elbows,hips, knees and other joints, and dental orthopraxy lines.

A number of methods for fabricating titanium and its alloys with adesired shape have been developed. However, the titanium alloy, forexample Ti-6Al-4V, is very difficult to be cold worked, i.e. thecold-worked titanium alloy has poorer mechanical properties or thecold-worked titanium alloy cracks after cold working.

Precision casting has the advantage that the cast produced has a nearnet shape, which greatly decreases the titanium fabrication cost. Also,precision casting is particularly suitable for producing objects with asmall volume, high size accuracy, and complicated shape, for example indental and osteological fields. Titanium is inherently difficult to castdue to its high melting point and high reactivity. Its low density isanother problem in casting.

U.S. Pat. No. 6,572,815B1 discloses a technique to improve thecastability of pure titanium by doping an alloying metal in an amount of0.01 to 3 wt %, preferably 0.5 to 3 wt %, and more preferably about 1 wt%. Among various alloying metals used in this application bismuth isfound the most promising element.

US patent publication No. 2004-0136859 A1 discloses a technique toimprove the castability of titanium alloys by doping an alloying metalin an amount of 0.01 to 3 wt %, preferably 0.5 to 3 wt %, and morepreferably about 1 wt % of bismuth, the disclosure of which isincorporated herein by reference.

U.S. Pat. No. 4,810,465 discloses a free-cutting Ti alloy. The basicalloy composition of this free-cutting Ti alloy essentially consists ofat least one of S: 0.001-10%, Se: 0.001-10% and Te: 0.001-10%; REM:0.01-10%; and one or both of Ca: 0.001-10% and B: 0.005-5%; and thebalance substantially Ti. The Ti alloy includes one or more of Ti—S (Se,Te) compounds, Ca—S (Se, Te) compounds, REM-S (Se, Te) compounds andtheir complex compounds as inclusions to improve machinability. Someoptional elements can be added to above basic composition. Alsodisclosed are methods of producing the above free-cutting Ti alloy and aspecific Ti alloy which is a particularly suitable material forconnecting rods. Bismuth up to 10% was suggested in this free-cutting Tialloy. However, there is no teaching as to the improvement ofcastability or reducing surface tension of pure titanium or a titaniumalloy.

U.S. Pat. No. 5,176,762 discloses an age hardenable beta titanium alloyhaving exceptional high temperature strength properties in combinationwith an essential lack of combustibility. In its basic form the alloycontains chromium, vanadium and titanium the nominal composition of thebasic alloy being defined by three points on the ternarytitanium-vanadium-chromium phase diagram: Ti-22V-13Cr, Ti-22V-36Cr, andTi-40V-13% Cr. The alloys of the invention are comprised of the betaphase under all the temperature conditions, have strengths much inexcess of the prior art high strength alloys in combination withexcellent creep properties, and are nonburning under conditionsencountered in gas turbine engine compressor sections. Bismuth up to1.5% was suggested in this age hardenable beta titanium alloy. However,there is no teaching as to the improvement of castability or reducingsurface tension of pure titanium or a titanium alloy.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a techniquecapable of making a titanium alloy article by cold working.

Another object of the present invention is to provide a techniquecapable of making a titanium alloy article by casting a titanium alloycomposition with an improved castability, and cold working.

Another object of the present invention is to provide a techniquecapable of making a titanium alloy article having an enhanced fatiguelife.

The present invention includes (but not limited to) the followingpreferred embodiments:

1. A method of making an article of a titanium alloy comprising

i) quenching a work piece, which is made of a titanium alloy compositioncomprising a) about 0.01-5 wt % Bi based on the weight of thecomposition; b) at least one alloy element selected from the groupconsisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having atemperature higher than a beta transition temperature of said titaniumalloy composition to a temperature lower than 500° C. at an averagecooling rate greater than 10° C./second between the beta transitiontemperature and 500° C., so that the quenched work piece contains Pphase with a body-centered cubic crystal structure as a major phase; andii) plastically deforming the quenched work piece.

2. The method of Item 1, wherein said cooling rate is greater than 25°C./second.

3. The method of Item 1, wherein the work piece to be quenched in stepi) has a thickness less than 1.0 cm.

4. The method of Item 1, wherein the work piece to be quenched in stepi) has a thickness less than 0.5 cm.

5. The method of Item 3, wherein said average cooling rate is greaterthan 25° C./second.

6. The method of Item 1 further comprising iii) heating the deformedwork piece to a temperature higher than 500° C.; and iv) cooling theheated deformed work piece.

7. The method of Item 1, wherein the work piece has a temperature of800-1200° C. before said quenching in step i).

8. The method of Item 1, wherein said plastically deforming in step ii)is carried out at room temperature.

9. The method of Item 1, wherein said titanium alloy compositioncomprises 0.1-3 wt % Bi.

10. The method of Item 9, wherein said titanium alloy compositionfurther comprises at least one eutectoid beta stabilizing elementselected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au,Pd, Si and Sn.

11. The method of Item 9, wherein said titanium alloy compositionconsists essentially of 0.1-3 wt % Bi, 10-50 wt % of at least one alloyelement selected from the group consisting of Mo, Nb, Ta, Zr and Hf,based on the weight of the composition, and the balance Ti.

12. The method of Item 11, wherein said titanium alloy compositionconsists essentially of 0.5-1.5 wt % Bi, 10-20 wt % of Mo, based on theweight of the composition, and the balance Ti.

13. The method of Item 11, wherein said titanium alloy compositionconsists essentially of about 1 wt % Bi, about 15 wt % of Mo, based onthe weight of the composition, and the balance Ti.

14. The method of Item 6 further comprising v) polishing the cooled workpiece so that a surface of the polished work piece has an averagesurface roughness less than about 0.1 μm.

15. The method of Item 1 further comprising polishing the plasticallydeformed work piece so that a surface of the polished work piece has anaverage surface roughness less than about 0.1 μm.

16. The method of Item 1, wherein said article is a dental casting.

17. The method of Item 1, wherein said article is a medical implant.

18. A method of making an article of a titanium alloy comprising

I) quenching a work piece, which is made of a titanium alloy compositioncomprising a) about 0.01-5 wt % Bi based on the weight of thecomposition; b) at least one alloy element selected from the groupconsisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having atemperature higher than a beta transition temperature of said titaniumalloy composition to a temperature lower than 500° C. at an averagecooling rate greater than 10° C./second between the beta transitiontemperature and 500° C., so that the quenched work piece contains βphase with a body-centered cubic crystal structure as a major phase; andII) polishing the quenched work piece so that a surface of the polishedwork piece has an average surface roughness less than about 0.1 μm.

19. The method of Item 18, wherein said titanium alloy compositionfurther comprises at least one eutectoid beta stabilizing elementselected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au,Pd, Si and Sn.

20. The method of Item 18, wherein said titanium alloy compositionconsists essentially of 0.1-3 wt % Bi, 10-50 wt % of at least one alloyelement selected from the group consisting of Mo, Nb, Ta, Zr and Hf,based on the weight of the composition, and the balance Ti.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the followingdrawings wherein:

FIG. 1 shows X-ray diffraction spectra of the specimens of Ti-15Mo-1Bialloy subjected separately to water quenching, liquid nitrogenquenching, air cooling and furnace cooling, at a scanning speed of3°/min;

FIG. 2 is a plot showing the average cooling rates between 1000-300° C.of the specimens of Ti-15Mo-1Bi alloy shown in FIG. 1;

FIG. 3 is a plot showing the bending strength of the specimens ofas-cast Ti-15Mo, as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi,cold-rolled/annealed (5 min at 650° C.) Ti-15Mo—Bi, cold-rolled/annealed(5 min at 750° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 850° C.)Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 900° C.) Ti-15Mo-1Bi, andcold-rolled/annealed (5 min at 950° C.) Ti-15Mo-1Bi;

FIG. 4 is a plot showing the elastic modulus of the specimens of as-castTi-15Mo, as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi,cold-rolled/annealed (5 min at 650° C.) Ti-15Mo-1Bi,cold-rolled/annealed (5 min at 750° C.) Ti-15Mo-1Bi,cold-rolled/annealed (5 min at 850° C.) Ti-15Mo-1Bi,cold-rolled/annealed (5 min at 900° C.) Ti-15Mo-1Bi, andcold-rolled/annealed (5 min at 950° C.) Ti-15Mo-1Bi;

FIG. 5 is a plot showing the ultimate tensile strength (UTS), yieldstrength (YS) and elongation of the specimens of as-cast Ti-15Mo-1Bi,as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650° C.)Ti-15Mo-1Bi, and cold-rolled/annealed (1 min at 900° C.) Ti-15Mo-1Bi;and

FIG. 6 is a plot showing the tensile modulus of the specimens of as-castTi-15Mo-1Bi, as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650°C.) Ti-15Mo-1Bi, and cold-rolled/annealed (1 min at 900° C.)Ti-15Mo-1Bi.

FIG. 7 is a plot showing the fatigue lives (numbers of cycles tofailure) of cold-rolled (78% reduction in thickness)/annealed (900° C.,1 min) Ti-15Mo-1Bi specimens with different surface roughness values at900 MPa loading.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the present application find that Ti-15Mo, though beinga beta phase alloy, cannot withstand excess cold metal working (plasticdeformation). With addition of 1 wt % Bi, not only the castability ofthe alloy is largely improved (as shown in US patent publication No.2004-0136859 Al application), it is discovered in this invention thatits workability (especially cold workability—which is very critical forindustrial application/fabrication) can also be dramatically improved.Further, this excellent cold workability is critically dependent on thecooling rate of Ti-15Mo—Bi alloy, i.e. the phase structure thereof.Therefore, the Ti-15Mo—Bi alloy must have a configuration, e.g.thickness, enabling a fast cooling rate to obtain the desired β phase ofthe cooled Ti-15Mo—Bi alloy. In one of the preferred embodiments of thepresent invention, the specimens of Ti-15Mo-1Bi alloy were heated to atemperature higher than its beta transition temperature (about 850° C.),and cooled with water quenching, liquid nitrogen quenching, air coolingand furnace cooling to provide different cooling rates, and X-raydiffraction (XRD) for phase analysis of the cooled specimens wasconducted.

Compared with Ti-6Al-4V (the most popularly-used Ti alloy for medicalimplant), Ti-15Mo-1Bi (cold-worked or cold-worked/annealed) has at leastfollowing advantages:

(a) More biocompatible (without Al and V—especially V).

(b) Ti-15Mo-1Bi exhibits excellent cold workability. On the other hand,Ti-6Al-4V cannot be cold-worked but has to be hot-worked (typically at900-1000° C.), that largely limits its applications and increasescomplexity and cost in processing.

(c) Thermomechanically-treated Ti-15Mo-1Bi demonstrates a similar (oreven higher) mechanical strength with an acceptable (for cold-workedalloy) or higher (cold-worked and annealed alloy) elongation.

Metal working methods include (but not limited to) such common methodsas rolling, forging, swaging, drawing, and extrusion, etc.

Preferred embodiments according to the present invention will bedescribed in the following examples that are intended as illustrativeonly since numerous modifications and variations therein will beapparent to those skilled in the art.

EXAMPLE 1

Ti-15Mo-1Bi alloy (15 wt % Mo and 1 wt % Bi) was prepared from acommercially pure titanium (c.p. Ti) bar, molybdenum of 99.95% andbismuth of 99.5% in purity using a commercial arc-meltingvacuum-pressure type casting system (Castmatic, Iwatani Corp., Japan).The melting chamber was first evacuated and purged with argon. An argonpressure of 1.5 kgf/cm² was maintained during melting. Appropriateamounts of the c.p. Ti bar, molybdenum and bismuth were melted in aU-shaped copper hearth with a tungsten electrode. The ingot wasre-melted three times to improve chemical homogeneity.

A specimen having an outer diameter of 7 mm and a length of 29 mm wasprepared from the Ti-15Mo-1Bi alloy, at one end of which was furtherprovided with a hole having a diameter of 3.5 mm and a depth of 12 mmfor mounting a K-type thermocouple therein. A titanium in the form of asponge was received in a quartz tube and fixed at a bottom thereof by aquartz cap, and the specimen equipped with the thermocouple was insertedinto the quartz tube and hermetically mounted inside the quartz tubewith one end of the thermocouple being connected to a temperaturerecorder (Sekonic SS-250F, Sekonic, Japan). The quartz tube at thesealed end was further equipped with a vacuum pump, and a vacuum meter.The quartz tube was vacuumed for five minutes, and placed in an airfurnace (S19, Nabertherm®, Germany) preheated at 1000° C. for 30minutes. The quartz tube was removed from the air furnace, and thespecimen together with the thermal couple was subjected to waterquenching, liquid nitrogen quenching or air cooling. The average coolingrates recorded are shown in FIG. 1 and Table 1, in which the averagecooling rate of a furnace-cooled specimen is also shown.

X-ray diffraction (XRD) for phase analysis of the cooled specimens wasconducted using a Rigaku diffractometer (Rigaku D-max IIIV, Rigaku Co.,Tokyo, Japan) operated at 30 kV and 20 mA. A Ni-filtered CuK_(α)radiation was used for this study. A silicon standard was used forcalibration of diffraction angles. Scanning speed of 3°/min was used.The phases were identified by matching each characteristic peak in thediffraction pattern with the JCPDS files. The results are shown in FIG.2, and are summarized in Table 1. TABLE 1 Average cooling rate, ° C./secPhase Water-quenched 211 β phase with a bcc* crystal structure LiquidN₂-quenched 26 β phase with a bcc crystal structure Air-quenched 9 βphase with a bcc crystal structure Furnace-cooled 0.05 α + β (α phasedominates)*body-centered cubic

EXAMPLE 2 Ti-15Mo and Ti-15Mo-1Bi Cold Rolling

Ti-15Mo (15 wt % Mo) and Ti-15Mo-1Bi (1-5 wt % Mo and 1 wt % Bi) alloyswere prepared from a commercially pure titanium (c.p. Ti) bar,molybdenum of 99.95% and bismuth of 99.5% in purity using a commercialarc-melting vacuum-pressure type casting system (Castmatic, IwataniCorp., Japan). The melting chamber was first evacuated and purged withargon. An argon pressure of 1.8 kgf/cm² was maintained during melting.Appropriate amounts of the c.p. Ti bar, molybdenum and bismuth weremelted in a U-shaped copper hearth with a tungsten electrode. The ingotwas re-melted three times to improve chemical homogeneity.

Specimens having a thickness of 5.0 mm, a width of 13 mm and a length of70 mm were prepared from the Ti-15Mo and i-15Mo-1Bi alloys using agraphite mold. The specimens removed from the mold were water-quenched,and surface-finished before being subjected to cold rolling. The coldrolling was carried our at room temperature using a 100-ton rollingmachine (VF PCAK-P1, Toshiba Corp., Japan), wherein the specimen wasrolled through a gap between two rollers several times with differentdeforming magnitudes by adjusting the gap.

When the cold rolling was conducted with reductions in thickness of 1.5mm, 0.9 mm, 0.9 mm, 0.3 mm and 0.3 mm in sequence (with a totalreduction in thickness of 78%), all the specimens of Ti-iSMo-1Bi couldbe cold-rolled to the final thickness (with a total reduction inthickness of 78%) without breaking down or showing any cracking on thesurfaces or edges of the specimens. However, the specimens of Ti-15Moeither showed deformation bands on the surfaces or cracking on the edgesof the specimens, or even broke down during rolling. When suchcold-rolled Ti-15Mo-1Bi specimens were bending-tested (using the samemethod as described in Example 3), all the specimens could be bent tothe preset deflection limit of 8 mm, while most of the cold-rolledTi-15Mo specimens failed prematurely. It can be understood from thisexample that the addition of 1 wt % Bi into Ti-15Mo alloy cansignificantly enhance the cold-rolling workability of the alloy.

EXAMPLE 3 Bending Strength and Elastic Modulus of Ti-15Mo andTi-15Mo-1Bi Alloys

Three-point bending tests were performed using a desk-top mechanicaltester (Shimadzu AGS-500D, Tokyo, Japan) operated at 0.5 mm/sec. Reducedsize (36×5×1 mm) specimens were cut from the-as-cast Ti-15Mo, as-castTi-15Mo-1Bi, and cold-rolled Ti-15Mo-1Bi. The cold-rolled Ti-15Mo-1Bispecimens were prepared according to the method described in Example 2.

Some of the Ti-15Mo-1Bi cold-rolled specimens were subjected to afurther heat treatment and water quenching before the bending test. Theheat treatment was conducted by sealing the specimen in a quartz tubeand at the sealed end was further equipped with a vacuum pump, and avacuum meter. The quartz tube was evacuated for five minutes, and placedin a tube-type furnace heated at a predetermined temperature for 5minutes. After the heat treatment, the quartz tube was removed from thefurnace, and the specimen was subjected to water quenching.

All the specimens for bending test were polished using sand paper to a#1000 level. The bending strengths were determined using the equation,σ=3PL/2bh ²,where σ is bending strength (MPa); P is load (Kg); L is span length (mm)(L=30 mm); b is specimen width (mm) and h is specimen thickness (mm).The modulus of elasticity in bending was calculated from the loadincrement and the corresponding deflection increment between the twopoints on a straight line as far apart as possible using the equation,E=L ³ ΔP/4bh ³Δδ,where E is modulus of elasticity in bending (Pa); ΔP is load incrementas measured from preload (N); and Δδ is deflection increment at mid-spanas measured from preload. The average bending strength and modulus ofelasticity in bending were taken from at least six tests under eachcondition.

The comparison of the bending strength and elastic modulus of theTi-15Mo as-cast specimens, the Ti-15Mo-1Bi as-cast specimens, and theTi-15Mo-1Bi cold-rolled specimens with and without heat treatment areshown in FIGS. 3 and 4.

It can be seen from the data shown in FIGS. 3 and 4 that the bendingproperties of the Ti-15Mo-1Bi alloy can be largely modified throughmechanical and/or thermal treatments. The cold-rolled Ti-15Mo-1Bispecimens with or without a further heat treatment have a bendingstrength higher and a comparable elastic modulus than/to the as-castTi-15Mo-1Bi specimens.

In addition to the above-mentioned 5-minute heat treatment conditions,short-term heat treatments including 3-minute, 1-minute and 0.5-minuteat 900° C. were also applied on the Ti-15Mo as-cast specimens before thebending test. The bending strength and elastic modulus of theTi-15Mo-1Bi as-cast specimens, the cold-rolled Ti-15Mo-1B specimens withand without heat treatment are listed in Table 2. TABLE 2 Bendingstrength and elastic modulus of the cold-rolled Ti—15Mo—1Bi specimenswith and without heat treatment Bending strength (MPa) Bending modulus(GPa) As-cast 1200 77 Ti—15Mo—1Bi cold-rolled 2200 76 Ti—15Mo—1Bicold-rolled 1480 84 Ti—15Mo—1Bi, 900° C., 3 min cold-rolled 1630 77Ti—15Mo—1Bi, 900° C., 1 min cold-rolled 1860 84 Ti—15Mo—1Bi, 900° C.,0.5 min

The data in Table 2 reveal that the cold-rolled Ti-15Mo-1Bi specimenshave the highest bending strength. With increasing the heat treatmenttime at 900° C., the bending strength of the specimens decreased.However, as will be shown in the tensile test data (Example 4), theductility of the alloy increases with increasing the heat treatmenttime.

EXAMPLE 4 Tensile Test of Ti-15Mo-1Bi Alloys

The tensile test was conducted on the reduced-size (40 mm in length, 10mm in width and 1 mm in thickness with 10 mm in gauge length and 3 mm ingauge width) as-cast Ti-15Mo-1Bi specimens, the cold-rolled Ti-15Mo-1Bspecimens without heat treatment, cold-rolled Ti-15Mo-1B specimens withheat treatments (650° C., 5 min; and 900° C., 1 min). A ShimadzuServopulser system (Shimadzu, Japan) with a crosshead speed of 0.5mm/min was used for the tensile test. The specimens were prepared andheat-treated as in Example 3. The results are shown in FIGS. 5 and 6.

As shown in FIG. 5, the Ti-15Mo-1Bi as-cast specimens have the lowestaverage ultimate tensile strength (UTS) of 819 MPa with an average yieldstrength (YS) of 560 MPa. The cold-rolled Ti-15Mo-1Bi specimens (78%reduction in thickness) have an average UTS of 1300 MPa, and an averageYS of 735 MPa, both of which decline after the heat treatments, but theUTS is still higher than that of the as-cast specimens. As to theelongation (%), the cold-rolled Ti-15Mo-1Bi specimens have the lowestaverage elongation of 6.7%, which is much lower than the as-castspecimens (about 30%). However, the heat treatment enables thecold-rolled Ti-15Mo-1Bi specimens more ductile, wherein the cold-rolledTi-15Mo-1Bi specimens with a heat treatment of 900° C., 1 min have anaverage elongation of 25.7%.

The average tensile modulus shown in FIG. 6 is changing from 78 GPa(as-cast) to 75 GPa (as-rolled), and to the lowest 65 GPa (cold-rolled,900° C., 1 min), indicating that the cold rolling does not significantlyaffect the tensile modulus, and the heat treatment will further decreasethe tensile modulus. (Note: a high strength and low modulus is oftendesirable for a medical implant.)

EXAMPLE 5 Bending Fatigue Test of Ti-15Mo-1Bi Alloy

The inventors of the present application have conducted a bendingfatigue test on the cold-rolled Ti-15Mo-1Bi specimens (78% reduction inthickness) with a heat treatment of (900° C., 1 min). A servo-hydraulictype testing machine (EHF-EG, Shimadzu Co., Tokyo, Japan) was used forthe fatigue test on smooth plate specimens with dimensions of 40 mm inlength, 5 mm in width and 1.5 mm in thickness. The smooth platespecimens were subjected to fatigue loading with a sinusoidal waveformat room temperature in air at a frequency of 4 Hz with a stress ratioR=0.1. Four different levels of surface roughness were prepared: (1)surface roughness of Ra=0.9-1.1 μm (the Ra value is measured accordingto ISO 4287: 2000 method) obtained from #60 sand paper; (2) surfaceroughness of Ra=0.1-0.2 μm obtained from #1000 sand paper; (3) surfaceroughness of Ra<0.1 μm obtained from #1500 sand paper, followed bymechanical polishing using 1 μm, 0.3 μm and 0.05 μm alumina powder insequence; and (4) surface roughness of Ra<0.1 μm obtained from #1500sand paper, followed by chemical polishing for 5 seconds in a solutioncontaining 5 vol % HF, 15 vol % HNO₃ and 80 vol % water.

It is discovered from the fatigue test data that the fatiguelife/fatigue resistance is critically dependent on the surface roughnessof the specimen being tested. As indicated in FIG. 7, the fatigue lives(numbers of cycles to failure) of all the five specimens prepared from#60 sand paper (Ra=0.9-1.1 μm) are between about 4×10³ and 10⁴ cycles;the fatigue lives of all the five specimens prepared from #1000 sandpaper (Ra=0.1-0.2 μm) are between about 10⁴ and 6×10⁴ cycles.

It is worth noting that the mechanically polished specimens and thechemically polished specimens (both with Ra<0.1 μm) have dramaticallyincreased fatigue lives. In each group, four out of six specimens testeddemonstrate fatigue lives longer than 10⁶ (specimens did not fail after10⁶ cycles). This result suggests that, for practical application, it iscritical for the Ti-15Mo-1Bi alloy to be prepared with a surfaceroughness of Ra<0.1 μm. Any cyclic load-bearing device made from thiskind of material with surface roughness larger than 0.1 μm can have arisk of premature fatigue failure.

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except as and to the extent that they are included in theaccompanying claims. Many modifications and variations are possible inlight of the above disclosure.

1. A method of making an article of a titanium alloy comprising i)quenching a work piece, which is made of a titanium alloy compositioncomprising a) about 0.01-5 wt % Bi based on the weight of thecomposition; b) at least one alloy element selected from the groupconsisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having atemperature higher than a beta transition temperature of said titaniumalloy composition to a temperature lower than 500° C. at an averagecooling rate greater than 10° C./second between the beta transitiontemperature and 500° C., so that the quenched work piece contains βphase with a body-centered cubic crystal structure as a major phase; andii) plastically deforming the quenched work piece.
 2. The method ofclaim 1, wherein said cooling rate is greater than 25° C./second.
 3. Themethod of claim 1, wherein the work piece to be quenched in step i) hasa thickness less than 1.0 cm.
 4. The method of claim 1, wherein the workpiece to be quenched in step i) has a thickness less than 0.5 cm.
 5. Themethod of claim 3, wherein said average cooling rate is greater than 25°C./second.
 6. The method of claim 1 further comprising iii) heating thedeformed work piece to a temperature higher than 500° C.; and iv)cooling the heated deformed work piece.
 7. The method of claim 1,wherein the work piece has a temperature of 800-1200° C. before saidquenching in step i).
 8. The method of claim 1, wherein said plasticallydeforming in step ii) is carried out at room temperature.
 9. The methodof claim 1, wherein said titanium alloy composition comprises
 0. 1-3 wt% Bi.
 10. The method of claim 9, wherein said titanium alloy compositionfurther comprises at least one eutectoid beta stabilizing elementselected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au,Pd, Si and Sn.
 11. The method of claim 9, wherein said titanium alloycomposition consists essentially of 0.1-3 wt % Bi, 10-50 wt % of atleast one alloy element selected from the group consisting of Mo, Nb,Ta, Zr and Hf, based on the weight of the composition, and the balanceTi.
 12. The method of claim 11, wherein said titanium alloy compositionconsists essentially of 0.5-1.5 wt % Bi, 10-20 wt % of Mo, based on theweight of the composition, and the balance Ti.
 13. The method of claim11, wherein said titanium alloy composition consists essentially ofabout 1 wt % Bi, about 15 wt % of Mo, based on the weight of thecomposition, and the balance Ti.
 14. The method of claim 6 furthercomprising v) polishing the cooled work piece so that a surface of thepolished work piece has an average surface roughness less than about 0.1μm.
 15. The method of claim 1 further comprising polishing theplastically deformed work piece so that a surface of the polished workpiece has an average surface roughness less than about 0.1 μm.
 16. Themethod of claim 1, wherein said article is a dental casting.
 17. Themethod of claim 1, wherein said article is a medical implant.
 18. Amethod of making an article of a titanium alloy comprising I) quenchinga work piece, which is made of a titanium alloy composition comprisinga) about 0.01-5 wt % Bi based on the weight of the composition; b) atleast one alloy element selected from the group consisting of Mo, Nb,Ta, Zr and Hf; and c) the balance Ti, having a temperature higher than abeta transition temperature of said titanium alloy composition to atemperature lower than 500° C. at an average cooling rate greater than10° C./second between the beta transition temperature and 500° C., sothat the quenched work piece contains β phase with a body-centered cubiccrystal structure as a major phase; and II) polishing the quenched workpiece so that a surface of the polished work piece has an averagesurface roughness less than about 0.1 μm.
 19. The method of claim 18,wherein said titanium alloy composition further comprises at least oneeutectoid beta stabilizing element selected from the group consisting ofFe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.
 20. The method of claim18, wherein said titanium alloy composition consists essentially of0.1-3 wt % Bi, 10-50 wt % of at least one alloy element selected fromthe group consisting of Mo, Nb, Ta, Zr and Hf, based on the weight ofthe composition, and the balance Ti.